CN106981976A - The method that T-shaped three-level three-phase inverter suppresses center line common mode current - Google Patents

Abstract

The present invention relates to a kind of method that T-shaped three-level three-phase inverter suppresses center line common mode current, useIt is used as the modulated signal for modulating T-shaped three-level three-phase inverter；V_aref、V_bref、V_crefThe respectively conventional sinusoidal wave pulse modulated signal of three-phase, V_offsetFor the triple-frequency harmonics offset signal of the conventional sinusoidal wave pulse modulated signal of three-phase, V_{offset_LC}To be injected into the voltage bias signal of modulated signal, V with the resonance current for suppressing LC wave filters_{offset_LC}=K_fi_{a_high}, K_fFor the conversion coefficient of electric current to voltage, i_{a_high}For the inverter side output current i of T-shaped three-level three-phase inverter_aHigh fdrequency component.The method of the present invention can realize the LC filter resonances for effectively suppressing to include in circulation center line and inverter inverter side electric current, reduce inverter common mode leakage current, improve inverter performance.

Description

Method for restraining neutral line common mode current of T-type three-level three-phase inverter

Technical Field

The invention relates to a method for suppressing neutral common mode current in a T-type three-level three-phase inverter.

Background

In a distributed power generation system, an inverter is used as a bridge for connecting renewable energy sources (such as photovoltaic power generation, wind power generation and the like) with a power grid or a load, and the performance of the inverter directly affects the whole distributed power generation system. Compared with a two-level inverter, a multi-level inverter in a distributed power generation system has the following advantages: (1) the output voltage and current harmonic waves of the inverter are smaller; (2) the output voltage change rate is smaller; (3) the output power is larger. Therefore, multilevel inverters are gaining attention and application in distributed power generation systems.

In the multi-level inverter, the T-type three-level three-phase inverter has the following advantages compared with the diode-clamped three-level three-phase inverter: (1) 6 power diodes are reduced, so that the system cost is reduced; (2) the switching frequency of the inverter is 5 kHz-30 kHz, and the T-shaped three-level three-phase inverter has higher efficiency than a diode-clamped three-level three-phase inverter. Therefore, the T-type three-level inverter is widely applied to a distributed power generation system.

In a photovoltaic power generation system, an inverter can be divided into an isolated inverter and a non-isolated inverter, and the non-isolated inverter is widely applied to a distributed photovoltaic power generation system due to the advantages of high efficiency, small size and the like. For a non-isolated T-type three-level three-phase photovoltaic grid-connected inverter, in order to reduce the common-mode leakage current of the photovoltaic grid-connected inverter, most commercial inverters generally adopt a scheme of connecting a common point of an output filter capacitor of the T-type three-level three-phase inverter to a neutral point (circulating current neutral line) of a direct-current bus capacitor, as shown in fig. 1. Therefore, a part of high-frequency current of the inverter can circulate on the neutral line, and the output common-mode leakage current of the inverter is greatly reduced. However, due to the existence of the circulating neutral line of the inverter, high-frequency components of the T-type three-level three-phase inverter, which form resonance by the LC filter, will also flow on the inverter and the circulating neutral line, affecting the inverter-side current and the circulating neutral line current of the photovoltaic grid-connected inverter, resulting in the output of the inverter-side current and the resonant current of the circulating neutral line containing the LC filter by the photovoltaic grid-connected inverter. Therefore, if the control is not performed, the high-frequency current of the circulating neutral line and the high-frequency component of the inverter side current are increased.

For most commercial photovoltaic grid-connected inverters at present, no consideration is given to suppression of the resonance current of the LC filter, and the main reason is that the resonance current of the LC filter mainly circulates on the inverter side and the circulation neutral line and does not circulate on the inverter grid side (does not flow into the power grid). Therefore, in order to restrain the circulating neutral line high-frequency current of the T-type three-level three-phase grid-connected inverter and the inverter side resonance high-frequency current of the inverter, the method for restraining the circulating neutral line current of the T-type three-level three-phase photovoltaic grid-connected inverter and improving the performance of the T-type three-level three-phase photovoltaic grid-connected inverter is invented, and the method has a good application prospect in a distributed power generation system.

Disclosure of Invention

The invention aims to provide a method for restraining neutral common mode current of a T-type three-level three-phase inverter, which can restrain the neutral common mode current of a grid-connected inverter so as to improve the performance of the inverter.

In order to achieve the purpose, the invention adopts the technical scheme that:

a method for restraining neutral common mode current of a T-shaped three-level three-phase inverter is applied to SPWM modulation and three-phase current signal i output by an inverter side_a、i_b、i_cIn the T-type three-level three-phase inverter, an output of the T-type three-level three-phase inverter is connected to a power grid through an LC filter, and a method for suppressing a neutral line common mode current of the T-type three-level three-phase inverter is as follows: by using

As a modulation signal for modulating the T-type three-level three-phase inverter;

wherein,three-phase modulation signals, V, for modulating the T-type three-level three-phase inverter, respectively_aref、V_bref、V_crefRespectively, three-phase conventional sine-wave pulse modulated signal, V_offsetA third harmonic offset signal, V, for said three-phase conventional sine wave pulse modulated signal_{offset_LC}For a voltage bias signal with suppression of the resonant current of the LC filter injected into the modulation signal, V_{offset_LC}＝K_fi_{a_high}，K_fFor the conversion coefficient of current to voltage, i_{a_high}Outputting current i to the inversion side of the T-shaped three-level three-phase inverter_aThe high frequency component of (2).

The inverter side of the T-shaped three-level three-phase inverter outputs current i_aObtaining its low-frequency current i through a low-pass filter_{a_low}Then i is_{a_high}＝i_a-i_{a_low}。

Wherein, ω is_fAnd s is a Laplace transform operator, which is the cut-off frequency of the low-pass filter.

V_offset＝-(max(V_aref,V_bref,V_cref)+min(V_aref,V_bref,V_cref))/2。

Three-phase current signal i output by the inversion side of the T-shaped three-level three-phase inverter_a、i_b、i_cRespectively transformed into DC components i under dq coordinate system_d、i_qThe respective corresponding DC components i of the three-phase current signals_d、i_qRespectively associated with given valuesAfter comparison, the output quantity is output and regulated through a proportional-integral regulator, and the regulated output quantity is subjected to coordinate transformation to obtain a three-phase conventional sine wave pulse modulation signal V_aref、V_bref、V_cref。

And acquiring the spatial angle of the power grid through a phase-locked loop during coordinate transformation.

The T-type three-level three-phase inverter adopts a power grid voltage directional vector control mode.

Due to the application of the technical scheme, compared with the prior art, the invention has the following advantages: the method can effectively inhibit the resonance of the circulation neutral line and the LC filter contained in the current of the inversion side of the inverter, reduce the common-mode leakage current of the inverter and improve the performance of the inverter.

Drawings

Fig. 1 is a schematic diagram of a resonant current circulation loop of an LC filter in a distributed power generation inverter system.

Fig. 2 is a schematic structural diagram of a distributed power generation inverter system.

FIG. 3 is a schematic diagram of a vector control method for directionally inhibiting circulating current neutral common mode current of grid voltage of a three-phase grid-connected inverter.

FIG. 4 is a vector diagram of a vector control system based on grid voltage orientation.

FIG. 5 is a steady-state simulation waveform diagram of a three-phase grid-connected inverter; (a) the traditional method circulates neutral current; (b) the method has the advantages that the current is in the loop current; (c) the traditional method inverts the three-phase current at the side; (d) the method of the invention inverts the three-phase current at the side; (e) in the traditional method, three-phase current at the power grid side is adopted; (f) the method of the invention is used for the three-phase current at the power grid side.

Detailed Description

The invention will be further described with reference to examples of embodiments shown in the drawings to which the invention is attached.

The first embodiment is as follows: referring to fig. 2, the distributed power generation inversion system is composed of a distributed renewable energy source, a T-type three-level three-phase inverter, an LC filter, a power grid, and the like. In this embodiment, the distributed power generation inverter system is a photovoltaic power generation inverter system, wherein the distributed renewable energy source is a photovoltaic array, and in fig. 2, the dc power source E is used for supplying power to the photovoltaic array_dcAnd a DC input resistor R_dcAnd (4) equivalence. T-type three-level three-phase inverter based on lightThe power input by the voltage array realizes active power and reactive power control, the LC filter filters high-frequency components of the output current of the inverter, and the output of the T-shaped three-level three-phase inverter is connected to a power grid through the LC filter. In FIG. 2, u_an、u_bn、u_cnThree-phase voltage output by T-type three-level three-phase grid-connected inverter, e_a、e_b、e_cFor three-phase mains voltage, i_a、i_b、i_cThe three-phase inverter is characterized in that three-phase output current is output by an inversion side of a T-shaped three-level three-phase inverter, L is a filter inductor, and C is a filter capacitor. And a common point of an output filter capacitor C of the T-type three-level three-phase inverter is connected to a neutral point of a direct-current bus capacitor.

The T-type three-level three-phase inverter adopts SPWM modulation. To improve the utilization of the dc bus voltage, the third harmonic is usually added in the conventional sine wave pulse width modulation (SPWM). The modulation signal of the inverter after the third harmonic is added is:

wherein, V_aref、V_bref、V_crefRespectively, three-phase conventional sine-wave pulse modulated signal, V_offsetA third harmonic bias signal V, which is a three-phase conventional sine wave pulse modulated signal_offsetExpressed as:

V_offset＝-(max(V_aref,V_bref,V_cref)+min(V_aref,V_bref,V_cref))/2 (2)

wherein the max function takes the maximum value of the variable, wherein the min function takes the minimum value of the variable.

In order to inhibit the resonant current of the LC filter from flowing on a circulating neutral line, the resonant current generated by the T-shaped three-level three-phase inverter is counteracted with the resonant current. Firstly, inverting side current i of T-shaped three-level three-phase grid-connected inverter_aThrough aA low-frequency current i of the inverter A phase current is obtained by a low-pass filter_{a_low}It can be shown as:

wherein ω is_fFor the cut-off frequency of the low-pass filter, in the present system ω_fIs selected as omega_f314.15 rad/s; s is the Laplace transform operator.

Then three-level three-phase inversion side current i_aHigh-frequency component current i_{a_high}Comprises the following steps:

i_{a_high}＝i_a-i_{a_low}(4)

the voltage bias signal with suppression of the resonant current injection of the LC filter into the modulation signal is then:

V_{offset_LC}＝K_fi_{a_high}(5)

wherein K_fFor current to voltage conversion factor, K in the system of the invention_f＝4。

Therefore, the following modulation signals with the circulating neutral-line common-mode current suppression function are adopted as modulation signals for modulating the T-type three-level three-phase inverter:

wherein,three-phase modulation signals, V, for modulating a T-type three-level three-phase inverter, respectively_aref、V_bref、V_crefRespectively, three-phase conventional sine-wave pulse modulated signal, V_offsetThird harmonic offset signal, V, being a three-phase conventional sine wave pulse modulated signal_{offset_LC}To have inhibition LCThe resonant current of the filter is injected into the voltage bias signal of the modulation signal.

The schematic diagram of the vector control method for directionally inhibiting the circulating current common mode current of the three-phase grid-connected inverter power grid voltage based on the control method is shown in the attached figure 3. Three-phase current signal i output by inversion side of T-type three-level three-phase inverter_a、i_b、i_cRespectively converted into DC component i under dq coordinate system by coordinate transformation (ABC/αβ/dq transformation)_d、i_qRespective corresponding DC components i of three-phase current signals_d、i_qRespectively associated with given values(given value)Freely given according to the magnitude of the power, e.g. given by the system as) After comparison, a Proportional Integral (PI) regulator is used for outputting regulated output quantity, and coordinate transformation (dq/αβ/abc transformation) is carried out on the regulated output quantity to obtain a three-phase conventional sine wave pulse modulation signal V_aref、V_bref、V_cref. In order to obtain the spatial angle of the power grid, the spatial angle of the power grid is obtained through a Phase Locked Loop (PLL) during coordinate transformation. And inverting side phase current i to suppress circulating neutral common mode current_aObtaining V through filtering and transformation_{offset_LC}To obtain the required three-phase modulation signal

In order to realize independent control of active power and reactive power of the T-type three-level three-phase grid-connected inverter, the three-level three-phase inverter adopts grid voltage orientation vectorAnd controlling the quantity. The grid Voltage oriented vector control (VOC) mode refers to dq synchronous rotation coordinate system and three-phase grid Voltage space vectorRotate synchronously and dq rotates the d-axis and voltage vector of the coordinate systemAnd (4) overlapping. The vector diagram of the vector control system based on the grid voltage orientation is shown in fig. 4.

From fig. 4, the three-phase grid voltage vectorComponent on d-axis of dq rotating coordinate systemComponent E on the q-axis_q0. According to the instantaneous power theory, the instantaneous active power P and the instantaneous reactive power Q output by the three-phase grid-connected inverter are respectively as follows:

wherein E_d、E_qD-axis components and q-axis components of the grid voltage in the dq rotation coordinate system are obtained; i.e. i_d、i_qAnd outputting d-axis components and q-axis components of current in the dq rotating coordinate system for the three-phase grid-connected inverter.

The voltage of the power grid is fixed on the d axis

The combination of formula (7) and formula (8) gives:

from the equation (9), the instantaneous active power P of the system and the component i of the inverter output current on the d-axis_dProportional to the instantaneous reactive power Q and the component i of the output current on the Q-axis_qIs in direct proportion. Thus, through i_dAnd i_qThe instantaneous active power and the reactive power of the system can be respectively controlled by the control of the controller.

In order to verify the correctness and effectiveness of the invented method, the control method of fig. 3 is used to perform comparative simulation on the invented method and the conventional method. The system simulation parameters are as in table 1.

TABLE 1 simulation parameters

Rated power	P＝10kW
		Filter inductor	L＝1.3mH
Filter capacitor	C＝4.7μF
		Bus filter capacitor	C1＝3000uF
Switching frequency of inverter	fs＝20kHz
		Direct current input	Edc＝650V
DC input resistor	Rdc＝1Ω
		Frequency of mains voltage	fg＝50Hz
Effective value of grid voltage	eRMS＝220V

For realizing inversion with power factor of 1, active power current is givenReactive power current settingFig. 5 shows a circulating neutral current, an inverter output inversion side three-phase current and a grid side three-phase current. FIGS. 5(a), (c), (e) are conventional, while FIGS. 5(b), (d), (f) are inventive. From the simulated waveforms of fig. 5, it is seen that: (1) circulating neutral current i of the invented method_cmIs 5A, while the line current i is conventionally measured_cmThe maximum value of (A) is 9A, and the current of the line in the circulating current output by the method is obviously smaller than that of the traditional method; (2) the quality of the waveform of the current output by the inversion side of the method is superior to that of the traditional method, and the current on the inversion side of the traditional method obviously contains high-frequency resonant current of an LC filter; (3) the inverter grid side current is basically the same in the invention method and the traditional method. As seen from fig. 5 comparing the simulation waveforms: the method realizes effective inhibition of the resonance of the circulating neutral line and the current LC filter on the inverter side of the inverter on the basis of not increasing any hardware, and improves the performance of the inverter. The method has good application value in a photovoltaic power generation system.

The invention provides a method for inhibiting resonance current of an LC filter of a T-shaped three-level three-phase photovoltaic grid-connected inverter and inhibiting neutral line common-mode current, which greatly reduces neutral line common-mode current high-frequency components of the inverter, is applied to a T-shaped three-level three-phase photovoltaic grid-connected power generation system, improves the performance of the T-shaped three-level three-phase photovoltaic grid-connected inverter and improves the performance of the photovoltaic grid-connected power generation system.

The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims (7)

1. A method for restraining neutral common mode current of a T-shaped three-level three-phase inverter is applied to SPWM modulation and three-phase current signal i output by an inverter side_a、i_b、i_cIn the T-type three-level three-phase inverter, an output of the T-type three-level three-phase inverter is connected to a power grid through an LC filter, characterized in that: the method for restraining the neutral line common mode current of the T-shaped three-level three-phase inverter comprises the following steps: by using

$\left\{\begin{array}{c}{V}_{aref}^{*}=V_{aref}+V_{offset}-V_{offset\_LC}\\ V_{bref}^{*}=V_{bref}+V_{offset}-V_{offset\_LC}\\ V_{cref}^{*}=V_{cref}+V_{offset}-V_{offset\_LC}\end{array}\right.$

As a modulation signal for modulating the T-type three-level three-phase inverter;

wherein,respectively three-phase modulation signals for modulating the T-type three-level three-phase inverter,

V_aref、V_bref、V_crefrespectively three-phase conventional sine wave pulse modulated signals,V_offseta third harmonic offset signal, V, for said three-phase conventional sine wave pulse modulated signal_{offset_LC}For a voltage bias signal with suppression of the resonant current of the LC filter injected into the modulation signal, V_{offset_LC}＝K_fi_{a_high}，K_fFor the conversion coefficient of current to voltage, i_{a_high}Outputting current i to the inversion side of the T-shaped three-level three-phase inverter_aThe high frequency component of (2).

2. The method for suppressing neutral common mode current in a T-type three-level three-phase inverter according to claim 1, wherein: the inverter side of the T-shaped three-level three-phase inverter outputs current i_aObtaining its low-frequency current i through a low-pass filter_{a_low}Then i is_{a_high}＝i_a-i_{a_low}。

3. The method for suppressing the common mode current of the neutral line of the T-type three-level three-phase inverter according to claim 2, characterized in that:wherein, ω is_fAnd s is a Laplace transform operator, which is the cut-off frequency of the low-pass filter.

4. The method for suppressing neutral common mode current in a T-type three-level three-phase inverter according to claim 1, wherein: v_offset＝-(max(V_aref,V_bref,V_cref)+min(V_aref,V_bref,V_cref))/2。

5. The method for suppressing neutral common mode current in a T-type three-level three-phase inverter according to claim 1, wherein: three-phase current signal i output by the inversion side of the T-shaped three-level three-phase inverter_a、i_b、i_cRespectively transformed into DC components i under dq coordinate system_d、i_qThe respective corresponding DC components i of the three-phase current signals_d、i_qRespectively associated with given valuesAfter comparison, the output quantity is output and regulated through a proportional-integral regulator, and the regulated output quantity is subjected to coordinate transformation to obtain a three-phase conventional sine wave pulse modulation signal V_aref、V_bref、V_cref。

6. The method for suppressing the common mode current of the neutral line of the T-type three-level three-phase inverter according to claim 5, wherein: and acquiring the spatial angle of the power grid through a phase-locked loop during coordinate transformation.

7. The method for suppressing neutral common mode current in a T-type three-level three-phase inverter according to claim 1, wherein: the T-type three-level three-phase inverter adopts a power grid voltage directional vector control mode.